Semiconductor device with asymmetric pocket implants

ABSTRACT

A semiconductor device ( 1 ) has a source ( 2 ) a gate ( 3 ) and a drain ( 4 ), a single deep-pocket ion implant ( 8 ) in a source-drain depletion region, and a single shallow-pocket ion implant ( 9 ) in the source-drain depletion region.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device with a singledeep-pocket ion implant and a single shallow-pocket ion implant tocounter short channel effects, SCE.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,344,405 discloses a semiconductor device that comprises,a source, an oxide-supported gate and a drain, which extend lengthwisealong a substrate. The device requires a threshold voltage V_(t) to turnon the device, from a nonconductive, off state. Leakage currents nearthe source and drain tend to occur within a source depletion region anda drain depletion region. These depletion regions are in the substratebeneath the oxide-supported gate, and between the source and the drain.The source and drain depletion regions electrically couple to thetransistor source and the transistor drain, respectively, whichcontributes to leakage current.

When the length of the gate, L_(gate), is made desirably smaller, forexample, when L_(gate) decreases below 0.1 μm (micrometers), thesedepletion areas have a tendency for electrically coupling to each other,which increases the likelihood of a leakage current between the sourceand the drain. Such an electrical coupling, as L_(gate) decreases, isreferred to as, a short channel effect (SCE), or alternatively as, apunch through effect.

SUMMARY OF THE INVENTION

The present invention counters the punch through effect or SCE byproviding a semiconductor device having a deep-pocket ion implant in adeep source-drain depletion region, and further having a shallow-pocketion implant in a shallow source-drain depletion region. Variousembodiments of the present invention counter variations in the punchthrough effect or SCE by having implants in different locations. Anotherembodiment of the present invention resides in a method of fabricating asemiconductor device to counter the punch through effect or SCE.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an embodiment of a semiconductor devicehaving shallow and deep pocket ion implants at a drain side of thesemiconductor device.

FIG. 2 is a schematic view of an embodiment of a semiconductor devicehaving shallow and deep pocket ion implants at a source side of thesemiconductor device.

FIG. 3 is a schematic view of an embodiment of a semiconductor devicehaving shallow and deep pocket ion implants, respectively, at a sourceside and a drain side of the semiconductor device.

FIG. 4 is a schematic view of another embodiment of a semiconductordevice having shallow and deep pocket ion implants, respectively, at adrain side and a source side, respectively, of the semiconductor device.

FIG. 5 is a schematic view of a lightly doped drain implant processstep.

FIG. 6 is a schematic view of a tilted pocket implant process step toproduce first and second ion implants.

FIG. 7 is a schematic view of a Si₃N₄ process step and an SiO₂ processstep.

FIG. 8 is a schematic view of two process steps providing spacers on asemiconductor device.

FIG. 9 is a schematic view of a process step of deep source-drainimplantation and a process step of rapid thermal annealing to provide anasymmetric ion implant when the ion strength concentration of a secondion implant is countered.

FIG. 10 is a schematic view of the process step of FIG. 5 applied to theembodiment of FIG. 4.

FIG. 11 is a schematic view of the process step of FIG. 6 applied to theembodiment of FIG. 4.

FIG. 12 is a schematic view of the process step of FIG. 6 applied asecond time to the embodiment of FIG. 4.

FIG. 13 is a schematic view of the two process steps of FIG. 7 appliedto the embodiment of FIG. 4.

FIG. 14 is a schematic view of the two process steps of FIG. 8 appliedto the embodiment of FIG. 4.

FIG. 15 is a schematic view of the two process steps of FIG. 9 appliedto the embodiment of FIG. 4.

FIG. 16 is a schematic view of the process step of FIG. 5 applied to theembodiment of FIG. 3.

FIG. 17 is a schematic view of the process step of FIG. 6 applied to theembodiment of FIG. 3.

FIG. 18 is a schematic view of the process step of FIG. 6 applied asecond time to the embodiment of FIG. 3.

FIG. 19 is a schematic view of the two process steps of FIG. 7 appliedto the embodiment of FIG. 3.

FIG. 20 is a schematic view of the two process steps of FIG. 8 appliedto the embodiment of FIG. 3.

FIG. 21 is a schematic view of the two process steps of FIG. 9 appliedto the embodiment of FIG. 3.

FIG. 22 is a schematic view of the process step of FIG. 5 applied to theembodiment of FIG. 1.

FIG. 23 is a schematic view of the process step of FIG. 6 applied to theembodiment of FIG. 1.

FIG. 24 is a schematic view of the process step of FIG. 6 applied asecond time to the embodiment of FIG. 1.

FIG. 25 is a schematic view of the two process steps of FIG. 7 appliedto the embodiment of FIG. 1.

FIG. 26 is a schematic view of the two process steps of FIG. 8 appliedto the embodiment of FIG. 1.

FIG. 27 is a schematic view of the two process steps of FIG. 9 appliedto the embodiment of FIG. 1.

FIG. 28 is a schematic view of the process step of FIG. 5 applied to theembodiment of FIG. 2.

FIG. 29 is a schematic view of the process step of FIG. 6 applied to theembodiment of FIG. 2.

FIG. 30 is a schematic view of the process step of FIG. 6 applied asecond time to the embodiment of FIG. 2.

FIG. 31 is a schematic view of the two process steps of FIG. 7 appliedto the embodiment of FIG. 2.

FIG. 32 is a schematic view of the two process steps of FIG. 8 appliedto the embodiment of FIG. 2.

FIG. 33 is a schematic view of the two process steps of FIG. 9 appliedto the embodiment of FIG. 2.

FIG. 34 is a schematic view of a semiconductor device having two sets ofsymmetric ion implants.

FIG. 35 is a graph of device performance compared with performance of adevice having symmetric halo implants.

FIG. 35 is a graph indicating SCE or punch through effect on deviceperformance as a function of gate length.

DETAILED DESCRIPTION

With reference to FIG. 1, an embodiment of the present invention, whichis indicated as Dd, resides in a semiconductor device 1 comprising: asource 2, an oxide-supported gate 3, supported on a film of oxide 3 a,and a drain 4, all of which are supported lengthwise on a semiconductingsubstrate 5.

The source 2 includes a shallow source extension 2 a and a deep sourceportion 2 b in a source-drain region. The drain 4 includes a shallowdrain extension 4 a and a deep drain portion 4 b in the source-drainregion. The source-drain region is in the substrate 5 under the gate 3.The source-drain region extends lengthwise to include the source 3 andthe drain 4. The source-drain region has a shallow source-drain regionbetween the source extension 2 a and the drain extension 4 a. Thesource-drain region has a deep source-drain region between the deepsource portion 3 a and the deep drain portion 4 a. For example, thesemiconductor device 1 is a field effect transistor, FET. An FETcomprises either a PMOSFET, a p-type metal oxide silicon field effecttransistor, or an NMOSFET, an n-type metal oxide silicon field effecttransistor.

The semiconductor device 1 has a source depletion region 6. The sourcedepletion region 6 includes a shallow source depletion region 6 a and adeep source depletion region 6 b. The semiconductor device 1 has a draindepletion region 7. The drain depletion region 7 includes a shallowdrain depletion region 7 a and a deep drain depletion region 7 b. Thesource-drain depletion region is between the source 3 and the drain 4.The source-drain depletion region is in the substrate 5 under the gate3, and is where the SCE or punch through effect can occur.

With further reference to FIG. 1, asymmetric pocket implants areprovided to counter the SCE or punch through effect. They are singlepocket implants that are asymmetric relative to a vertical central axisof the semiconductor device 1, as viewed in FIG. 1. The semiconductordevice 1 has a deep-pocket ion implant 8 in the deep source-draindepletion region. A shallow-pocket ion implant 9 is in the shallowsource-drain depletion region. For a PMOSFET, both the deep-pocket ionimplant 8 and the shallow-pocket ion implant 9 are N-doped withrespective concentrations of N-doped ions. The respective concentrationsof N-doped ions are either equal or different. For an NMOSFET, both thedeep-pocket ion implant 8 and the shallow-pocket ion implant 9 areP-doped with respective concentrations of P-doped ions. The respectiveconcentrations of P-doped ions are equal or different. The structuredescribed above with reference to FIG. 1 applies similarly to each ofthe embodiments disclosed in FIGS. 2-4.

The embodiment of FIG. 1 discloses the deep pocket implant 8 at thedrain side, meaning, closer to the drain 4 than to the source 2. Thedeep pocket implant 8 at the drain side is in the drain depletion region7. The shallow pocket ion implant 9 is at the drain side, and in thedrain depletion region 7.

FIG. 2 discloses another embodiment of the present invention that isindicated as Ss. With reference to FIG. 2, the deep-pocket ion implant 8is at the source side, meaning closer to the source 2 than to the drain4. The deep-pocket ion implant 8 is in the source depletion region 6.The shallow-pocket ion implant 9 is in the substrate 5 at the sourceside and in the source depletion region.

FIG. 3 discloses another embodiment of the present invention that isindicated as Sd. With reference to FIG. 3, the deep-pocket ion implant 8is at the drain side, and in the drain depletion region 7. The shallowpocket implant 9 is at the source side, and in the source depletionregion 6.

FIG. 4 discloses another embodiment that is indicated as sD in FIG. 4.With reference to FIG. 4, the deep pocket ion implant 8 is at the sourceside, and in the source depletion region 6. The shallow-pocket ionimplant 9 is at the drain side, and in the drain depletion region 7.

FIGS. 1-4 disclose the asymmetric deep pocket implant 8 and theasymmetric shallow pocket implant 9 in different locations to providecountering adjustments that counter the variations in the punch througheffect or SCE.

With reference to FIGS. 5-9, a process of sequential process steps willnow be described for making a semiconductor device 1 with an asymmetricpocket implant. First, conventional process steps form the substrate 5,and an oxide film under the gate 3. Further conventional process stepsform the gate 3.

FIG. 5 discloses a process step of lightly doped drain and sourceimplantation. This process step is a shallow ion implantation at zerotilt angle. Implanted ion impurities form light doped source and drainregions of a transistor. For example implanted Arsenic, As, forms lightdoped source and drain regions for NMOS transistors. Further, forexample, Boron and/or Boron Fluoride, B and BF₂, impurities form lightdoped source and drain regions for PMOS transistors. The impurities areimplanted with a zero tilt angle. The impurities are impelled verticallyalong the vertical vector 10 in FIG. 5. For example, the light dopedsource 2 a and the light doped drain 4 a are formed simultaneously bythe first process step.

FIG. 6 discloses a process step of deep pocket ion implantation at atilt angle to form a set of two deep pocket ion implants 8. Implantedion impurities are formed simultaneously in the substrate 5. The tiltangle is tilted toward the source side of the semiconductor device 1 toform a primary implant 8 at the source side of the semiconductor device1, and form a secondary implant 8 at the drain side of the semiconductordevice 1.

The primary implant 8 is formed under the gate 3 on either the sourceside or the drain side, depending on the direction of the tilt angle.Although not shown, the tilt angle can be tilted to form a drain side,primary implant 8, at the drain side of the semiconductor device 1,wherein such primary implant 8 is under the gate 3 at the drain side ofthe semiconductor device 1.

A secondary implant 8 is formed at the drain side of the semiconductordevice. This secondary implant 8 will be countered by performance ofsuch further process steps to be discussed with reference to FIGS. 7-9.According to the invention, the primary implant 8 becomes a single ionimplant and an asymmetric ion implant when the secondary implant 8 iscountered.

A process step disclosed by FIG. 6 applies to implantation of eithershallow or deep ion implants 8 or 9, depending on the strength of theimplantation energy to impel the implants either shallow or deep, andfurther depending on the ion concentrations that dissipate theimplantation energy. Further, the tilt of the implant angle from shallow(larger tilt angle) to steep (smaller tilt angle) affects the implantdepth from shallow to deep, respectively.

The process step of FIG. 6 is capable of being repeated to form anotherset of primary and secondary shallow-pocket ion implants 9. For example,the process of FIG. 6 can be repeated to form a set of two shallow ionimplants 9, to be further disclosed by FIG. 12.

FIG. 7 discloses a first process step of forming a thin layer 11 of SiO₂an insulating barrier on the substrate 5 and on each gate 3 on thesubstrate. The thin layer 11 of SiO₂ is formed by TEOS(Tetraethoxysilane), by way of example. Another process step of forminga thicker layer 12 of Si₃N₄ on the insulating barrier covers the lateralsides of each gate 3. The layer 12 is formed by chemical vapordeposition, CVD, by way of example.

FIG. 8 discloses a process of forming spacers 13 of Si₃N₄ materialagainst the lateral sides of each gate 3. The spacers 13 are formed bydry etching the layer of Si₃N₄ and the spacers 13 are against aninsulating thin layer 11 of SiO₂ on the sides of the gate 3. Portions ofSiO₂ material are uncovered by the spacers 13. A process step of wetdipping in an etchant of SiO₂ removes the uncovered SiO₂ material.

FIG. 9 discloses a process step of deep source-drain implantation atzero tilt angle. This process step is a deep ion implantation at zerotilt angle of highly doped ions. Implanted ion impurities form a deepsource portion 2 b and a deep drain portion 4 b. For example implantedArsenic, As, forms implanted n+ source and n+ drain regions for NMOStransistors. Further, for example, Boron and/or Boron Fluoride, B andBF₂, impurities form implanted p+ source and p+ drain regions for PMOStransistors.

With continued reference to FIG. 9, the gate 3 and the spacer 13 coverthe source side, primary implant 8 while ions of the deep source portion2 b are impelled along the vector 10 of zero tilt angle. The drain side,secondary implant 8 is uncovered by the gate 3 and any spacer 13 whileions of the deep drain portion 4 b are impelled along the vector 10 ofzero tilt angle. Thus the ions of the deep drain portion 4 b arecombined with the uncovered secondary implant 8 in the space occupied bythe uncovered secondary implant 8. The doped polarity of the source 2and drain 4, including the portions 2 a, 2 b, 4 a and 4 b, are oppositethe doped polarity of the implants 8. For example, an Arsenic ion of thedeep drain portion 4 b has more free electrons than the single electronreceptor of a Boron ion of the uncovered secondary implant 8. Thus, thepolarity of the Boron ions in the secondary implant 8 is neutralized bythe Arsenic ions. Further, the secondary implant 8 is surrounded by theion distribution of the deep drain portion 4 b. The secondary implant 8is said to be countered by counter doped implanted ions of the deepdrain portion 4 b. Thus, the source side, primary implant 8 becomes anasymmetric ion implant, when the drain side, secondary implant 8 iscountered by counter doped implanted ions.

With continued reference, to FIG. 9, a process step of rapid thermalannealing of the semiconductor device 1 is performed, to refine thecrystalline structure and repair structural damage caused by ionimplantation. Annealing causes joining of the source extension 2 a andthe deep source 2. Annealing further causes joining of the drainextension 4 a and the deep drain 4.

FIGS. 10-15 disclose how the process steps of FIGS. 5-9 apply to makethe embodiment sD disclosed in FIG. 4. FIG. 10 discloses a light dopedsource 2 a and a light doped drain 4 a made by performing the processstep disclosed by FIG. 5.

FIG. 11 discloses a deep pocket primary implant 8 formed by performingthe process step of FIG. 6, with a tilt angle tilted to the source sideof the semiconductor device 1. FIG. 11 discloses a deep pocket secondaryimplant 8 that is countered by ion implants to be further disclosed byFIG. 15.

FIG. 12 discloses a drain side, shallow pocket primary implant 9 and ashallow pocket secondary implant 9 formed by performing the process stepof FIG. 6, by having the tilt angle tilted toward the drain side of thesemiconductor device 1. The primary implant 9 is under the gate 3.

FIG. 13 discloses a thin layer of SiO₂ and a thicker layer of Si₃N₄formed by performing the two process steps disclosed by FIG. 7.

FIG. 14 discloses spacers 13 formed by the two process steps disclosedby FIG. 8.

FIG. 15 discloses a deep source portion 2 b and a deep drain portion 4 bformed by the process step disclosed by FIG. 9. In FIG. 15, the sourceside, deep primary implant 8 and the drain side, shallow primary implant9 become asymmetric ion implants, respectively, when the secondaryimplant 8 and the secondary implant 9 are countered. Then a process ofrapid thermal annealing of the semiconductor device 1 is performed asdisclosed by FIG. 9.

FIGS. 16-21 disclose how the process steps of FIGS. 5-9 apply to makethe embodiment Sd disclosed by FIG. 3. FIG. 16 discloses a light dopedsource 2 a and a light doped drain 4 a made by performing the processstep disclosed by FIG. 5.

FIG. 17 discloses a drain side, deep pocket primary implant 8 formed byperforming the process step of FIG. 6, with a tilt angle tilted to thedrain side of the semiconductor device 1. FIG. 17 discloses a deeppocket secondary implant 8 that is countered by ion implants to bedisclosed by FIG. 21.

FIG. 18 discloses a source side, shallow-pocket primary implant 9 and adrain side, shallow-pocket secondary implant 9 formed by performing theprocess step of FIG. 6, with the tilt angle tilted toward the sourceside of the semiconductor device 1. The primary implant 9 is under thegate 3 at the source side of the semiconductor device 1.

FIG. 19 discloses a thin layer of SiO₂ and a thicker layer of Si₃N₄formed by the two process steps disclosed by FIG. 7.

FIG. 20 discloses spacers 13 formed by the two process steps disclosedby FIG. 8.

FIG. 21 discloses a deep source portion 2 b and a deep drain portion 4 bformed by the process disclosed by FIG. 9. In FIG. 21, the drain side,deep primary implant 8 and the source side, shallow primary implant 9become asymmetric ion implants, respectively, when the secondary implant8 and the secondary implant 9 are countered. Then a process of rapidthermal annealing of the semiconductor device 1 is performed asdisclosed by FIG. 9.

FIGS. 22-27 disclose how to make the embodiment Dd disclosed by FIG. 1.FIG. 22 discloses a shallow source extension 2 a and a shallow drainextension 4 a made by performing the process step disclosed by FIG. 5.

FIG. 23 discloses a drain side, deep pocket primary implant 8 formed byperforming the process step of FIG. 6, with a tilt angle tilted to thedrain side of the semiconductor device 1. FIG. 23 discloses a deeppocket secondary implant 8 that is countered by ion implants to bedisclosed by FIG. 27.

FIG. 24 discloses a drain side, shallow pocket primary implant 9 and ashallow pocket secondary implant 9 formed by performing the process stepof FIG. 6, by having the tilt angle tilted toward the drain side of thesemiconductor device 1. The drain side, primary implant 9 is under thegate 3 at the drain side of the semiconductor device 1.

FIG. 25 discloses a thin layer of SiO₂ and a thicker layer of Si₃N₄formed by the two process steps disclosed by FIG. 7.

FIG. 26 discloses spacers 13 formed by the two process steps disclosedby FIG. 8.

FIG. 27 discloses a deep source portion 2 b and a deep drain portion 4 bformed by the process disclosed by FIG. 9. In FIG. 27, the drain sideprimary deep implant 8 and the drain side primary shallow implant 9become asymmetric ion implants, respectively, when the deep secondaryimplant 8 and the shallow secondary implant 9 are countered. Then aprocess of rapid thermal annealing of the semiconductor device 1 isperformed, as disclosed by FIG. 9.

FIGS. 28-33 disclose how the process steps of FIGS. 5-9 apply to makethe embodiment Ss as disclosed by FIG. 2. FIG. 28 discloses a lightdoped source 2 a and a light doped drain 4 a made by performing theprocess step disclosed by FIG. 5.

FIG. 29 discloses a source side, deep pocket primary implant 8 formed byperforming the process step of FIG. 6, with a tilt angle tilted to thesource side of the semiconductor device 1. FIG. 29 discloses a deeppocket secondary implant 8 that is countered by ion implants to bedisclosed by FIG. 33.

FIG. 30 discloses a source side, shallow pocket primary implant 9 and ashallow pocket secondary implant 9 formed by performing the process stepof FIG. 6, by having the tilt angle tilted toward the source side of thesemiconductor device 1. The source side, primary implant 8 is under thegate 3 at the source side of the semiconductor device 1.

FIG. 31 discloses a thin layer of SiO₂ and a thicker layer of Si₃N₄formed by the two process steps disclosed by FIG. 7.

FIG. 32 discloses spacers 13 formed by the two process steps disclosedby FIG. 8.

FIG. 33 discloses a deep source portion 2 b and a deep drain portion 4 bformed by the process disclosed by FIG. 9. In FIG. 33, the source side,deep primary implant 8 and the source side, shallow primary implant 9become asymmetric ion implants, respectively, when the deep secondaryimplant 8 and the shallow secondary implant 9 are countered. Then aprocess of rapid thermal annealing of the semiconductor device 1 isperformed as disclosed by FIG. 9.

FIG. 34 discloses an embodiment SD of a semiconductor device 1 with aset of two deep-pocket primary implants 8, and a set of twoshallow-pocket primary implants 9. Further, the implants are known as,symmetric halo implants, meaning that they are imbedded at the samelevel, and further, they are symmetrically spaced apart. The embodimentSD is manufactured with symmetric halo implants as proposed by U.S. Pat.No. 6,344,405 and FR 2796204.

According to each of the embodiments disclosed in FIGS. 1-4, theshallow-pocket ion implant 9 has both, a smaller size and a heavier iondose, in contrast to the deep-pocket ion implant 8 having a larger sizeand smaller dose of ions. The shallow-pocket ion implant 9 is implantedby a lower level of implanting energy, in contrast to the deep-pocketion implant 8 being implanted by a higher level of implanting energy.For a PMOSFET, the deep-pocket ion implant 8 is doped with Arsenic (As)impurity. Each of the embodiments disclosed by FIGS. 1-4 aremanufactured with ion implants 8 and 9 according to the following Table.The Table further discloses the device of FIG. 34 manufactured withsymmetric halo implants. TABLE I Implant Energy Implant Dose Tilt AngleFrom Embodiment/pocket (KeV) (10¹³ cm⁻²) Vertical (Degrees) Dd/deep-pocket 30 1.0 15 Dd/ shallow pocket 20 1.2 30 Ss/ deep-pocket 301.0 15 Ss/ shallow pocket 20 1.2 30 Sd/ deep-pocket 30 1.0 15 Sd/shallow pocket 20 1.5 30 sD/ deep-pocket 30 1.0 15 sD/ shallow pocket 201.5 30 SD/ deep-pocket 30 1.5 15 SD/ shallow pocket 20 2.5 30

FIG. 35 discloses device performance of each of the embodiments Dd, Ss,Sd and sD of FIGS. 1-4, as compared with the device performance of thedevice SD of FIG. 34, wherein device stand-by current I_(off) innano-Amps per micrometer is plotted against device driving currentI_(dsat) in micro-Amps per micrometer.

FIG. 36 discloses the SCE or punch through effect on device performanceas a function of gate length L_(gate) in nanometers. The SCE or punchthrough effect is indicated for each of the embodiments Dd, Ss, Sd andsD of FIGS. 1-4, as compared with the device performance of the deviceSD of FIG. 34, wherein V_(t) roll-off, V_(t, sat) in Volts is plottedagainst reductions in gate length.

As indicated by FIGS. 35 and 36, symmetric halo implants of theembodiment SD are associated with the following drawbacks. They tend toincrease the apparent sheet resistance of the source and the drain,which decreases the driving current, I_(dsat), when the semiconductor isin a conducting state. Further, they tend to induce a parasitic junctioncapacitance at both the source-substrate junction and thedrain-substrate junction, which degrades the circuit impedance andsignal transmission speed. Further, leakage current in the deletion areaat both the source-substrate junction and the drain-substrate junctionis increased by the presence of the halo implants, which increases lostpower consumption and heat generation. Further, as gate length,L_(gate), is desirably reduced to make a smaller device, the thresholdvoltage V_(t) correspondingly decreases, which is an undesired effectknown as, V_(t) roll-off.

Symmetric halo implants are always confined to being in a symmetricalarrangement with respect to each other, and thereby, are unsuccessful incountering V_(t) roll-off. In contrast, the respective locations of thesingle asymmetric deep-pocket ion implant 8, and the single asymmetricshallow-pocket ion implant 9 of embodiments Dd, Ss, Sd and sD are chosenindependently, which individually adjusts the locations of the pocketimplants 8 and 9 to counter V_(t) roll-off. For example, FIG. 36indicates that V_(t) roll-off is countered effectively by the embodimentsD, FIG. 4, of the present invention, wherein the deep-pocket ionimplant 8 is at or near the source 2, and further wherein, theshallow-pocket ion implant 9 is at or near the drain 4.

The embodiments disclosed herein, as well as, further embodiments andmodifications of the present invention, are intended to be covered bythe spirit and scope of the appended claims.

1. A semiconductor device comprising: a source, a gate and a drain; asource-drain depletion region in a substrate under the gate; a singledeep-pocket ion implant in the source-drain depletion region; and asingle shallow-pocket ion implant in the source-drain depletion region.2. The semiconductor device as recited in claim 1 wherein; thedeep-pocket ion implant is asymmetric with respect to a central axis ofthe semiconductor device.
 3. The semiconductor device as recited inclaim 1 wherein; the shallow-pocket ion implant is asymmetric withrespect to a central axis of the semiconductor device.
 4. Thesemiconductor device as recited in claim 1 wherein; both the deep-pocketion implant and the shallow-pocket ion implant are asymmetric withrespect to a central axis of the semiconductor device.
 5. Thesemiconductor device as recited in claim 1 wherein; the deep pocket ionimplant is at a drain side; and the shallow pocket ion implant is at asource side.
 6. The semiconductor device as recited in claim 1 wherein;the deep pocket ion implant is at a source side; and the shallow pocketion implant is at a source side.
 7. The semiconductor device as recitedin claim 1 wherein; the deep pocket ion implant is at a drain side; andthe shallow pocket ion implant is at a drain side.
 8. The semiconductordevice as recited in claim 1 wherein; the deep pocket ion implant is ata source side; and the shallow pocket ion implant is at a drain side. 9.The semiconductor device as recited in claim 1, further comprising: asecondary deep-pocket ion implant countered by ions of the source. 10.The semiconductor device as recited in claim 1, further comprising: asecondary shallow-pocket ion implant countered by ions of the source.11. The semiconductor device as recited in claim 1, further comprising:a secondary deep-pocket ion implant countered by ions of the drain. 12.The semiconductor device as recited in claim 1, further comprising: asecondary shallow-pocket ion implant countered by ions of the drain. 13.The semiconductor device as recited in claim 1, further comprising: asecondary deep-pocket ion implant countered by ions of the source; and asecondary shallow-pocket ion implant countered by ions of the source.14. The semiconductor device as recited in claim 10, further comprising:a secondary deep-pocket ion implant countered by ions of the drain; anda secondary shallow-pocket ion implant countered by ions of the drain.15. The semiconductor device as recited in claim 10, further comprising:a secondary deep-pocket ion implant countered by ions of the source; anda secondary shallow-pocket ion implant countered by ions of the drain.16. The semiconductor device as recited in claim 1, further comprising:a secondary deep-pocket ion implant countered by ions of the drain; anda secondary shallow-pocket ion implant countered by ions of the source.17. A method of fabricating a semiconductor device with asymmetric ionimplants comprising the steps of: implanting primary and secondarydeep-pocket ion implants in a source-drain depletion region; implantingprimary and secondary shallow-pocket ion implants in the source-draindepletion region; and countering the secondary deep-pocket ion implantand the secondary shallow-pocket ion implant.
 18. The method of claim 17and further comprising the steps of: implanting ions to form the sourceand drain; countering the secondary deep pocket implant with ionsforming the drain; and countering the secondary shallow pocket implantwith ions forming the source.
 19. The method of claim 17 and furthercomprising the steps of: implanting ions to form the source and drain;countering the secondary deep pocket implant with ions forming thesource; and countering the secondary shallow pocket implant with ionsforming the drain.
 20. The method of claim 17 and further comprising thesteps of: implanting ions to form the source and drain; countering thesecondary deep pocket implant with ions forming the source; andcountering the secondary shallow pocket implant with ions forming thesource.
 21. The method of claim 17 and further comprising the steps of:implanting ions to form the source and drain; countering the secondarydeep pocket implant with ions forming the drain; and countering thesecondary shallow pocket implant with ions forming the drain.